1. Field of the Invention
The present invention generally relates to a mobile communication system, and more particularly to an apparatus and method for detecting and combining frequency errors.
2. Description of the Related Art
Wireless communication systems are mobile communication systems using a cellular communication scheme. These mobile communication systems make use of multiple access schemes for simultaneous communication with multiple users. Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) and frequency division multiple access (FDMA) are typically used as the multiple access schemes. With the rapid development of CDMA technology, a CDMA system is developing from a voice communication system into a system capable of transmitting high-rate packet data.
Mobile communication systems are classified into synchronous mobile communication systems and asynchronous mobile communication systems. The asynchronous mobile communication system has been adopted in Europe and the synchronous mobile communication system has been adopted in the United States. The mobile communication system for use in Europe is referred to as a Universal Mobile Telecommunication System (UMTS). A mobile communication terminal for use in the UMTS is referred to as a user equipment (UE).
In the mobile communication systems, a frequency offset is a factor of unavoidable performance deterioration since a carrier frequency gradually varies with temperature. Automatic Frequency Control (AFC) is required for compensating for the frequency offset. In the mobile communication systems for high-rate packet data, a pilot signal is set to a reference signal of a Frequency Error Control (FEC) loop.
An average phase of the pilot signal can be computed during a pilot signal transmission interval. The phase variation can be computed from a continuous pilot signal since a pilot channel transmits an unmodulated signal. That is, coordinates of a currently received symbol can be estimated by performing an integrate & dump (I&D) process for a received signal during a pilot signal interval. The phase variation of the currently received symbol is computed from the coordinates of the currently received symbol and coordinates of a previously received symbol. A computed value acts as a linear estimation value of the low phase variation and is proportional to a frequency error. The phase variation occurs because a Mobile Station (MS) has relatively inaccurate timing as compared to a Base Station (BS) as a first reason and because a Doppler shift is made due to movement of a UE, that is, an MS, as a second reason.
FIG. 1 is a schematic diagram illustrating an example of an apparatus for estimating and combining frequency errors in a conventional MS.
Referring to FIG. 1, the MS includes an antenna 110 for transmitting a data signal to and receiving a data signal from a BS, a radio frequency (RF) processor 120 for low-noise amplifying and frequency downconverting the data signal received from antenna 110 or frequency upconverting and amplifying the signal to be transmitted to a wireless network, a reference clock or voltage-controlled temperature compensated crystal oscillator (VCTCXO) 160 for providing a reference frequency and a searcher (not illustrated in FIG. 1) for performing a basic search function. The MS further includes a controller 125 for allocating signals to fingers in order to demodulate detected signals. That is, the controller 125 allocates the signals to a finger-1 AFC section 130-1 to a finger-N AFC section 130-N.
The finger AFC sections 130-1 to 130-N include a despreader 131 for despreading an allocated signal, an accumulator 133 for accumulating the despread signal and a frequency error detector or cross product frequency difference detector (CPFDD) 135 for detecting a frequency error from the accumulated signal, respectively.
Outputs of the finger AFC sections 130-1 to 130-N are input to an AFC combiner 140. The AFC combiner 140 receives and combines frequency errors from the finger AFC sections 130-1 to 130-N and provides a combined output to an AFC converter 150. The AFC converter 150 is connected to an output path of the AFC combiner 140 and converts an accumulated frequency error component into an analog signal. That is, the AFC converter 150 converts the output of the AFC combiner 140 into the analog signal for controlling the reference clock 160.
The frequency error detectors 135 detect frequency errors from pilot signals received via independent fading paths. The AFC combiner 140 adjusts gain by combining the frequency errors and providing a combined output to a loop filter (not illustrated). At this time, independent frequency errors time-delayed due to multipath of a channel are also combined, such that the multipath diversity effect may be obtained and the performance of a control loop may be improved.
Next, a movement direction and finger allocation of an MS according to a BS in a mobile communication system for providing a high-rate packet data service will be described with reference to FIG. 2.
Referring to FIG. 2, a finger-1 205 and a finger-2 207 are allocated to receive data from a BS-A 201 in a cell 210 where the data is transmitted from one BS. A finger-3 209 and a finger-4 211 are allocated to receive media access control (MAC) information from neighbor BSs (for example, a BS-B 203 and a BS-C 213). When moving quickly from the BS-A 201 of the cell 210 to the BS-B 203 of a cell 230, the conventional MS may obtain the multipath diversity effect with respect to timing errors between the MS and the BSs by combining all the fingers (for example, the finger 1 to the finger 4), thereby improving the performance of a control loop.
The AFC combiner 140 may estimate the timing errors by combining all paths. However, the compensation of a Doppler shift may be cancelled out when paths of multiple BSs are combined. The Doppler shift needs to be considered since a Doppler effect increases as a spreading factor (SF) decreases in a mobile communication system for providing a high-rate data service.